The world is a diverse place. People come in all shapes and sizes, with obvious differences in abilities and appearances. There are many 'faces of Man' found throughout the world. Yet, although the faces are different, the genetic structure, the DNA, is not so different as most people believe. Over 99% of the DNA in all people is identical. This is not an accident. Most of the proteins that carry out the biochemical functions of a human body are made exactly the same from one person to the next. The remaining one percent, however, accounts for people's diversity. It can also be used as a tool to determine how genetically "related" people are.
This 'sameness' has been of great interest to scientists ever since the discovery of DNA and its function in inheritance patterns. These scientists thought that perhaps the study of DNA could provide essential clues in the long-standing quest for an understanding of the evolution of Homo sapiens sapiens.
By comparing mutations in the DNA of people who live in different parts of the world, geneticists are developing new theories about how humans populated the Earth. The evidence points to a common African origin. Much of the work has been based upon maternal lines. The DNA of present-day Africans is more diverse than that of people of other continents, indicating that humans have lived there the longest. Traces of ancient African genes can be found in everyone living today.
Who were our ancestors? A stumbling block for this research has been the basic process of meiosis--the recombination of genetic factors from both the male and female of the species. Within this genetic mixing bowl, genetic clues could be lost. However, there is one form of DNA that is inherited from only the female; this DNA could be traced back in time, to perhaps the ancestor of us all! (Eve?)
Modern technologies, much like those used in the OJ Simpson trial, have made this quest possible. Research the "Eve Theory", match your wits with scientists and supercomputers, and decide whether or not you believe!
Cells are the fundamental working units of every living system. All the instructions needed to direct their activities are contained within the chemical DNA (deoxyribonucleic acid).
DNA from all organisms is made up of the same chemical and physical components. The DNA sequence is the particular side-by-side arrangement of bases along the DNA strand (e.g., ATTCCGGA). This order spells out the exact instructions required to create a particular organism with its own unique traits.
The genome is an organism’s complete set of DNA. Genomes vary widely in size: the smallest known genome for a free-living organism (a bacterium) contains about 600,000 DNA base pairs, while human and mouse genomes have some 3 billion. Except for mature red blood cells, all human cells contain a complete genome.
DNA in the human genome is arranged into 24 distinct chromosomes--physically separate molecules that range in length from about 50 million to 250 million base pairs. A few types of major chromosomal abnormalities, including missing or extra copies or gross breaks and rejoinings (translocations), can be detected by microscopic examination. Most changes in DNA, however, are more subtle and require a closer analysis of the DNA molecule to find perhaps single-base differences.
Each chromosome contains many genes, the basic physical and functional units of heredity. Genes are specific sequences of bases that encode instructions on how to make proteins. Genes comprise only about 2% of the human genome; the remainder consists of noncoding regions, whose functions may include providing chromosomal structural integrity and regulating where, when, and in what quantity proteins are made. The human genome is estimated to contain 30,000 to 40,000 genes.
Although genes get a lot of attention, it’s the proteins that perform most life functions and even make up the majority of cellular structures. Proteins are large, complex molecules made up of smaller subunits called amino acids. Chemical properties that distinguish the 20 different amino acids cause the protein chains to fold up into specific three-dimensional structures that define their particular functions in the cell.
The constellation of all proteins in a cell is called its proteome. Unlike the relatively unchanging genome, the dynamic proteome changes from minute to minute in response to tens of thousands of intra- and extracellular environmental signals. A protein’s chemistry and behavior are specified by the gene sequence and by the number and identities of other proteins made in the same cell at the same time and with which it associates and reacts. Studies to explore protein structure and activities, known as proteomics, will be the focus of much research for decades to come and will help elucidate the molecular basis of health and disease.
YOUR TASK (whether you choose to accept it or not!):
In your research, 'surf the net' in a quest for the answer to the essential question, "Does the inheritance pattern of mitochondrial DNA justify the Eve Theory?". In order to conduct an appropriate search, several questions must be answered. Those questions are:
1. What is a mitochondrion?
2. Where are mitochondria located?
3. What is mitochondrial DNA?
4. Is mitochondrial DNA different from nuclear (or other forms) of DNA?
5. What is the inheritance pattern of mitochondrial DNA?
6. Is the inheritance pattern of mitochondrial DNA different from the inheritance patterns of other forms of DNA?
7. Who is (or was) Eve?
8. What is the Eve Theory?
You may want to use the following sources, but you will want to search beyond these links to discover additional information to aid you in your conclusion.
Check these sources:
DNA From the Beginning
A comprehensive overview, organized around key concepts. The science behind each concept is explained by: animation, image gallery, video interviews, problem, biographies, and links.
The Great DNA Hunt
This abstract from 'Archaeology', a professional journal, offers information of a 'mitochondrial eve'. In addition, inheritance questions concerning the origins of the Japanese people are discussed. This site offers an interesting human migratory graphic.
The Debate over Human Origins
Information regarding the research question, as well as data concerning basic primate evolution, is contained in this site. The computer program used in the construction of the "Eve Theory" is discussed. A site well worth visiting, especially for those students concerned with the validity of the use of statistical analysis in the development of scientific theory.
Tracking Down Mitochondrial "Eve" and Y Chromosomal "Adam"
Mitochondrial Eve - An Explanation
Mitochondrial Eve represents one of the most poorly understood scientific ideas of the 20th century. In its time it has been feted and condemned, frequently by the same parties at different times. The premise of the idea is that we can all be traced back to a single woman living in Africa.
What are Mitochondria?
Mitochondria are the powerhouse of the cell. Technically 'organelles', mitochondria are essentially symbiotes1 – they consume the sugars that our bodies have converted from food, and in return produce electricity with which to power the cell. But why is it considered separate from the cell? This is an important distinction. The reason why mitochondria are considered separate is because they have their own DNA – DNA which is unaffected by other genetic exchanges.
Why is Mitochondrial DNA important?
Well, put it this way. You are a genetic mix of your parents, each of which contributed half of your genetic material. They, of course, have gone through the same process and share an equal split of their parent's DNA. This means if you stop this back-tracking process with your grandparents, you are already a genetic mix of six distinct individuals who may have come from different regions of the planet.
But one factor remains constant – the mitochondrial DNA hasn't altered at all – it remains intact through the female line. Male sperm contains only enough mitochondria to power the sperm to the surface of the egg – it does not enter the egg. The egg, however, contains mitochondria that have been passed from mother to daughter for countless generations. The only way for mitochondrial DNA to alter is by natural mutations, which occur very slowly when compared with the almost frantic gene mixing we and our parents take part in.
How Does This Relate to an 'Eve' Concept?
Because the rate of mitochondrial genetic mutation is slow, it can be used as a clock to turn back time to a period before the mutations had crept in. When mitochondrial DNA from certain populations in Africa are sampled, they can be compared with European mitochondrial DNA. The mutation difference between the two populations can then be compared, and a 'clock' can be produced, enabling the rate of mutation in mitochondria to be established. This produces a time-scale which indicates when modern Europeans first left Africa.
The genetic survey that produced the whole Mitochondrial Eve scenario didn't just sample Africans and Europeans – it sampled genes from people all over the planet. When mitochondrial DNA was compared, the survey discovered a startling result. Fundamental similarities in mitochondrial DNA in living humans suggested that we all contain genetic material from a single woman who was living in Africa.
That's Ridiculous – How Could a Single Being Populate a Planet?
And this is where the confusion sets in. A single organism can't populate a planet (arguments about amoeba aside). The evidence didn't suggest a single woman living in isolation from members of her own species. What it suggested was a genetic bottleneck – a period in human history when the population was so small that the genetic expressions of a single woman could have an impact on all humans living on the planet today.
She didn't live alone – she would have lived within a community. She didn't just pump babies out, either. There is no reason to suppose that she had more than one female child. But there is reason to suppose that whatever female children she had, they contained specific advantages for survival over the rest of the population.
But How Would the Population Become So Depleted?
Truth is, nobody knows the causes of the population crash. It could have been due to environmental pressures or a raging plague. The other important thing to consider is why did Eve survive and prosper where so many others died? Isolation is a tempting route to go down. She missed what-ever happened to everyone because she and her people where somehow isolated from the general population. Perhaps her people lived in a geographically remote valley, emerging when the threat had passed. Perhaps her people had access to food when starvation was rife – it is almost certain we will never know the cause for the depletion of the general population. A more fruitful line of inquiry is to question which traits made her progeny so successful.
Why Was She So Successful?
The reasons are all around you. What makes us so successful? An ability to share ideas, to help one another in dire circumstances, a certain creative flair to overcome everyday problems. Or perhaps she introduced the ability to slaughter those who came between us and required resources. We, as her children, display all of these traits. It could have been something as simple as wanderlust – a yearning to see what lay over the horizon. They were perhaps more fertile, were more agile, more resistant to disease, or could throw missiles more accurately than anyone else around at that time. If you want to find out, then next time you're on a bus, or train, or walking down the street, look around you – look at the behavior of your extended family.
Is Any of This True?
Well, yes and no. To get a completely accurate result the tests would have to be performed on every single person living on the planet today. The dates are in dispute, but the date is perhaps the least important point. Broadly speaking, populations do pass through bottlenecks. Eve had many ancestors – it helps if you think about her as an hourglass – she was the pinch in the glass through which our genes ran. There had been many more Eves before her, she is just our most recent common ancestor. There will probably be more population bottlenecks and more Mitochondrial Eves in the future.
Locating "Adam" and "Eve"
The rate at which polymorphisms develop though generations is known. Scientists can determine the number of polymorphisms in a certain population, as well as how many polymorphisms exist between different populations. In 1987, Mark Stoneking and Allan Wilson at the University of California, Berkeley, announced they had tracked down mitochondrial "Eve". By examining polymorphisms contained in the mitochondria, they were able to construct a global family tree. At its top branch was a woman who lived in Africa many years ago.
The next place to start looking for patterns of heredity was on the Y chromosome. Y chromosomal "Adam," the ancestor of all men, was determined in 1997. Two different research groups, led by Peter Underhill at Stanford University and Mike Hammer at the University of Arizona, each announced that Y chromosomal "Adam" had also lived in Africa many years ago. This is the most recent common male ancestor to all men in the world.
Anthropologists now commonly use mitochondrial and Y chromosomal DNA to track different populations. Recent studies of the Y chromosome from the lab of Nestor Bianchi in Argentina suggests that there is a common male ancestor to most of the Native American males of both North and South America. Two other research groups, led by Fabricio Santos in Brazil and Tatyiana Karafet at the University of Arizona, have shown evidence that this Native American "Adam" lived in Siberia.
There have also recently been studies on the relative migration rates of men and women. Mark Seielstad at Harvard University has determined that between different populations, there is much more similarity in mitochondrial DNA than in the Y chromosomal DNA. This means that women moved and migrated more often than men, a view dissimilar to the common "Man the Conqueror" idea.
It is clear that analysis of DNA polymorphisms has become a very powerful tool for anthropologists and historians. It should be noted, however, that the history of population migrations is not simple. Although "Adam" and "Eve" lived in Africa, scientists believe that their offspring migrated to Asia only to eventually return again. DNA analysis, however, provides a useful tool for anthropologists trying to determine how we got where we are now. Here is a collection of websites for further information on this exciting new technique.
* DNA Learning Center Solving the Mystery of the Romanovs
* Related Articles "America's Founding Fathers May Have Siberian Roots" - Daily InScight - May 1999
* Offline References Gibbons, Ann. 1997. "Y Chromosome Shows That Adam Was an African." Science October 31; 278: 804-805.
* Braginski, Ricardo, and Diego Hurtado de Mendoza. 1999 "Y Chromosomes Point to Native American Adam." Science March 5; 283: 1439-1440.
* Stoneking, Mark. 1998 "Women on the move." Nature Genetics November 20; 20:219-220.
A Different Twist: Mitochondria can be inherited from both parents
23 August 23, 2002 -- NewScientist.com news service
Mitochondria may not be inherited solely through the maternal line, according to new research that promises to overturn accepted biological wisdom.
If confirmed by other researchers, the findings could have huge implications for evolutionary biology and biochemistry.
Robert Sanders Williams, from Duke University Medical Center in North Carolina, says the findings are "remarkable and unanticipated. This is more than a mere curiosity. It asserts the principle that it can occur in humans. It could have significant implications for the study of human evolution and the migrations of populations," he says.
For decades biologists have assumed that mitochondria - the cells' power stations - are inherited solely through the maternal line.
Mitochondria in the sperm from the father were presumed to be destroyed immediately after conception, leaving behind only those from the mother. But Marianne Schwartz and John Vissing from the University Hospital Rigshospitalet in Copenhagen, have discovered that one of their patients inherited the majority of his mitochondria from his father.
"Even with very sensitive methods, paternal mitochondrial DNA has never been detected in man before," Schwartz told Reuters. "There are many examples of family pedigrees that follow mitochondrial diseases through the maternal line."
The pair made the discovery while trying to discover why one of their patients suffered extreme fatigue during exercise. The 28-year-old man had an entirely normal heart and lungs and his muscles appeared healthy. But on closer inspection, Schwartz and Vissing discovered that his muscles absorbed very little oxygen.
This led them to examine the genetic sequence of his mitochondria. They discovered two mutations in his mitochondrial DNA - one of which was responsible for his extreme fatigue.
To try and investigate the mutations further, they also sequenced the DNA of his mother, father and uncle. To their surprise, the sequence matched those of his father and uncle.
Muscle biopsies showed that about 90 per cent of his mitochondria came from his father. However, the mitochondria in his blood, hair roots and fibroblasts came entirely from his mother.
The two mutations appear to have arisen spontaneously during, or shortly after, conception.
The researchers think inheritance of paternal mitochondrial DNA is probably very rare. But the findings will have implications for a number of branches of biology. Evolutionary biologists often date the divergence of species by the differences in genetic sequences in mitochondrial DNA. Even if paternal DNA is inherited very rarely, it could invalidate many of their findings. It will also have implications for scientists investigating inherited metabolic diseases.
Journal reference: New England Journal of Medicine (vol 347, p576)
The use of mitochondrial mtDNA to investigate human history is not without drawbacks.
The rate of mtDNA mutation is not well known. A study by Parsons et al. (1997) found a rate 20 times higher than that calculated from other sources. In an article reviewing mtDNA research, Strauss (1999a) reports that mtDNA mutation rates differ in some groups of animals, and can even vary dramatically in single lineages. Although there are many agreements, some divergence dates for modern animals calculated from mtDNA do not match with what is known from the fossil record. There are suggestions from a few sources that paternal mtDNA can sometimes be inherited, which could affect analyses based on mtDNA.
In 1999 Awadalla et al. published a study suggesting that mtDNA could sometimes be inherited from fathers. If mtDNA is inherited only from mothers, the correlation between different mutations should not depend on how far apart on the genome they were. Instead, their measurements showed that mutations at distant sites on the mtDNA genome were less likely to be correlated than nearby mutations, suggesting that mtDNA from mothers and fathers could sometimes get mixed. However, there is no explanation so far as to how this recombination could be occurring, and the possibility that other phenomena could be causing this effect has not yet been disproved. If it occurs, mixing would mean that the dates from current mtDNA studies would be too old. If mixing is common enough, it could even mean that there was no mitochondrial Eve, because different parts of the mtDNA molecule would have different histories. (Awadalla et al. 1999, Strauss 1999b) Other studies, however, have contradicted these results and argued for strictly maternal mtDNA inheritance (Elson et al. 2001).
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